To buy: Sigma 85mm 1.4 vs. Canon 85mm 1.8?

I know it's not really fair to compare these two lenses, as the Sigma 85mm 1.4 is more often compared to the 85L. However, I have a purchase decision to make, and I can't quite decide... I'm an amateur and frankly have no need for the 85L. The only reason I'm considering the Sigma is because the price range is at least within reason for my purposes.

I only have a T1i (so just a prosumer body). Lenses: (two kit lenses: 17-55 and 55-250), 50mm f/1.8, and the 24-105 f/4L. So I would like to add a fast prime, one that'd mainly be used for portraits and indoor fast-action (so faster AF).

The reasons I'm debating between the 1.4 and 1.8:

1. Pro Sigma: much better IQ, incl. much more improved from any of my current lenses; faster lens.

2. Con Sigma: seems to be a toss up on whether I'll get a good copy; lack of micro adjustment on my camera may therefore be an issue; more expensive.

3. Pro Canon 1.8: cheaper; seems to be more consistent in every aspect; faster AF, so better for indoor action/sports.

4. Con Canon 1.8: IQ doesn't seem to be a huge step up (esp. wide open) from my current lenses (I know it's not fair to compare at f/1.8 to the f/4 lens, but I figure anything above f4, I am more likely to use my 24-105 instead of the 85mm); because I have a 50mm 1.8, I am not tempted to get another 1.8 and would prefer something faster.

Am I very wrong in my assessment? (criticism welcomed, but be kind ). I know the 50mm and 85mm have slightly different uses, but I feel like I won't get as much out of having two 1.8 lenses (conversely, would it be preferable to get the 85mm 1.8, and then the 50mm 1.4 and ditch the 50mm 1.8? I rather not, but it has crossed my mind).

Lastly, Adorama has a deal on the Sigma right now. Anyone know if it's easy to exchange with them if the copy I get has focusing issues? I know Amazon is good about such exchanges, but am unsure about Adorama (I know they are very reputable, but unsure of their exchange policies).